1. Field of the Invention
The present invention relates to an engine control system for controlling an engine operating condition such as a fuel injection amount or an ignition timing.
2. Description of the Related Art
Known in the prior art is a D-J type fuel injection system for an internal combustion engine wherein an intake pressure sensor is arranged in an intake line of the engine at a position downstream from a throttle valve for detecting an intake pressure as a parameter of an engine load. The detection of the intake pressure and an engine speed enables the detection of an amount of intake air introduced to the cylinder bore. A fuel injection amount is determined by the detected intake air amount, to maintain a designated air-fuel ratio value, and this amount of fuel is injected from the injector. This D-J type fuel injection system is advantageous in that the size of the sensor can be reduced, allowing a decrease in the air flow resistance, compared with an L-J type fuel injection system wherein a relatively large air flow meter is arranged in an intake passageway for detecting the intake air amount.
Contrary to the L-J type fuel injection system, this D-J type fuel injection system detects the amount of air introduced into the engine indirectly from the value of the intake pressure. This means that the sensor has the same output level value for the amount of newly introduced air when only air is introduced into the engine and when a gas, for example, an exhaust gas, other than the air is introduced into the engine. Therefore, when the exhaust gas re-circulation operation is carried out, it is necessary to compensate the detected output value of the sensor, to obtain a correct amount of new air introduced into the engine, if the map is appropriate for EGR operation. To this end, a system is disclosed in Japanese Unexamined Patent Publication No. 55-75548 wherein a fixed dimension orifice is arranged in an exhaust gas re-circulation passageway, and a pressure sensor is arranged for detecting a pressure drop across the orifice. This detected pressure drop is utilized for correcting the output value of the intake pressure sensor, and thereby to determine a precise value of the newly introduced air.
Nevertheless, this improved system has a drawback in that the precise amount of the new air cannot be determined, since it is not possible to directly detect the amount of new air. This causes a drawback in that a quick control of an air-fuel ratio of the target air-fuel ratio can not be obtained during a transient state of the engine.